When a voltage is applied to a metal oxide material, a phenomenon occurs in which the metal oxide material obtains two states, that is, a low resistance state and a high resistance state depending on the resistivity before voltage application and the magnitude of the applied voltage. An ReRAM (Resistance Random Access Memory) using this resistance change phenomenon as a storage medium (memory) has received a great deal of attention.
As for the device structure of the ReRAM, a three-dimensional cross-point structure including memory cells arranged at the intersections of word lines and bit lines has been proposed from the viewpoint of increasing the degree of integration. In the three-dimensional cross-point structure, at the time of write to a selected memory cell, a reverse bias is applied to unselected memory cells as well. For this reason, a storage element (variable resistance element) and a diode (rectifying element) having rectifying properties arranged as a memory cell. As the rectifying element, for example, an Si-PIN diode including a p-type impurity doped Si film (P), an Si film doped with no impurity or an Si film lightly doped with an impurity (I), and an n-type impurity doped Si film (N) is used.
One important thing of these functions is the variable resistance element that causes resistance change. As the variable resistance element, many metal oxide materials have been proposed. When changing the resistance state of a metal oxide film, a voltage is applied to move (diffuse) the metal element contained in the electrode material into the metal oxide film. A filament (conductive region) made of the metal element is thus formed in the metal oxide film, and the variable resistance element changes from the high resistance state to the low resistance state.
At this time, if the film thickness of the metal oxide film is 20 nm or more, the metal element diffuses into the entire metal oxide film. Even if the film thickness of the metal oxide film is 20 nm or less, the filament is difficult to form if the ionization of the metal element, is insufficient. Hence, when changing the variable resistance element to the low resistance state, a filament having a large size (for example, a width of 10 nm or more) is formed in the metal oxide film. Hence, the filament size largely varies depending on the place (for example, for each cell). Especially, when the memory cells are microfabricated to a width (diameter) of 10 nm or less and have a high density, some of them fail in forming the filament. As a result, the resistance value in the low resistance state may vary between the memory cells.
In addition, since the metal element diffuses into the entire metal oxide film in the low resistance state, it is difficult to move the metal element backward to obtain the high resistance state. For this reason, repetitive use may make even the resistance value in the high resistance state vary and may also decrease the resistance change amount between the low resistance state and the high resistance state.
Note that a structure using a single-layered GeSe film or a stacked film of an a-Si (amorphous silicon) film and an SiOX film in place of the metal oxide film has been proposed as the variable resistance element. However, these structures may also have the same phenomenon.